The construction of roadways, parking lots and driveways requires much aggregate material in the nature of crushed stone having various particle sizes. Some of this stone is used for base material and some is combined with asphalt cement to produce asphalt concrete. Still other crushed stone is combined with Portland cement and water for use in constructing concrete pavement and in building construction. Consequently, there is a great demand for crushed stone for use as aggregate materials in paving and building products.
Crushed stone used as aggregate material has three principal functions. It provides a relatively low-cost filler for cement-based products and asphalt concrete. It also provides a mass of particles that will resist the action of applied loads, abrasion, and the percolation of moisture and water. Finally, it minimizes volume changes in the pavement or building product resulting from the setting and hardening process and from moisture and temperature changes. The performance of these functions by the aggregate material depends on the mineral character of the aggregate as related to strength, elasticity and durability, and on the surface characteristics and gradation of the aggregate particles, particularly as related to workability and bonding within a hardened mass, and the density of the product.
Much of the crushed stone used as aggregate materials for roadway and building construction is blasted from quarry walls and processed into various aggregate sizes. Some quarries have permanent processing plants, and others employ portable processing plants for crushing, screening and separating the aggregate materials into various size fractions. In the operation of an open-pit quarry, it is common for holes to be drilled adjacent a wall or face of the quarry pit and for explosive charges to be placed in these holes. The explosives are detonated in order to loosen and remove stone for further processing. A large loader is employed to load off-road trucks with the stone for transport to the processing plant. The stone placed in these off-road trucks will include many large pieces that must be crushed, but it will also include stone that has been reduced to small particulate by the blasting process.
Frequently, an aggregate processing plant will include one or more items of reducing equipment such as jaw crushers, gyratory crushers, cone crushers, vertical shaft impact crushers or horizontal shaft impact crushers. Typically, an aggregate processing plant will also include one or more vibratory and/or grizzly screens to separate the aggregate materials into fractions of varying particle sizes. In addition, an aggregate processing plant will usually include one or more conveyors to transfer aggregate material for processing or to stockpile processed aggregates. Some aggregate processing plants are fixed installations. Others comprise portable processing plants that are mounted on trailers, or on a self-propelled chassis driven by wheeled or track drive systems. Portable processing plants are typically more compact than those comprising fixed installations.
One type of portable processing plant includes a crusher and a crusher feed conveyor to feed material to the crusher. It also includes a crusher collection conveyor under the crusher that delivers crushed material to a vibratory screen assembly, a transfer conveyor for conveying material too large to pass through the vibratory screen assembly to the crusher, and a plurality of discharge conveyors for separating material passing through the screens of the screen assembly. Such a processing plant is typically moved to a suitable location in the quarry for processing of the stone that is delivered by the off-road trucks. However, when the processing plant is loaded with stone obtained from a blasting process, all of the aggregate material, including portions that have already been reduced in particle size, is loaded into the crusher. This causes unnecessary wear on the crusher and reduces the speed at which material is processed. Furthermore, when all of the material is processed through the crusher, the screen assembly must be sized to process material having a top size that is dictated by the top size setting of the crusher.
One conventional way to avoid the unnecessary wear on the crusher incurred when all of the material to be processed is loaded into the crusher is to add a pre-screen component to remove the smaller sized aggregate material that does not need to be processed through the crusher.
It would be desirable, however, if the conventional processing equipment could be altered in such a way that it could be operated without processing all of the aggregate material through the crusher and without the addition of a separate pre-screen component.
Notes on Construction
The use of the terms “a”, “an”, “the” and similar terms in the context of describing the invention are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The terms “substantially”, “generally” and other words of degree are relative modifiers intended to indicate permissible variation from the characteristic so modified. The use of such terms in describing a physical or functional characteristic of the invention is not intended to limit such characteristic to the absolute value which the term modifies, but rather to provide an approximation of the value of such physical or functional characteristic.
Terms concerning attachments, coupling and the like, such as “connected” and “interconnected”, refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both moveable and rigid attachments or relationships, unless specified herein or clearly indicated by context. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship.
The use of any and all examples or exemplary language (e.g., “such as” and “preferably”) herein is intended merely to better illuminate the invention and the preferred embodiment thereof, and not to place a limitation on the scope of the invention. Nothing in the specification should be construed as indicating any element as essential to the practice of the invention unless so stated with specificity.